The present invention relates to a mobile radio telephone system, and more particularly a circuit for generating dual-tone multi-frequency (DTMF) signals and high/low-tone signals used in an advanced mobile phone system (AMPS).
AMPS employs the DTMF signals and the high/low-tone signals respectively as the end-to-end signals and the side tone signals. The end-to-end signals are automatically transmitted from a mobile vehicle telephone system to a local telephone exchange office or another party through the transmitter in the system. The side tone signal is to inform the user of the state of the vehicle telephone system, which is delivered to the user through a receiving low-frequency terminal, a control means and a handset in the system.
Generally, the keyboard of the AMPS is mounted in the handset. On the keyboard are arranged a few functional keys (for example SEND, END and CLR, etc.) together with the dialing keys. The dialing keys, as in a conventional multi-tone telephone, are arranged in the form of a matrix comprising four rows each designating one frequency of the low-frequency group, namely, 697 Hz (corresponding to the keys 1,2,3), 770 Hz (corresponding to the keys 4,5,6), 852 Hz (corresponding to the keys 7,8,9) and 941 Hz (corresponding to the keys *,0,#), and three columns each designating one frequency of the high-frequency group, namely, 1209 Hz (corresponding to the keys 1,4,7,*), 1336 Hz (corresponding to the keys 2,5,8,0), 1477 Hz (corresponding to the keys 3,6,9,#). When a particular dialing key is put on, the AMPS produces as the end-to-end signal a sine wave signal composed of a particular frequency of the low-frequency group and a particular frequency of the high-frequency group corresponding to the key. This is called the DTMF signal.
On the other hand, when one of the functional keys is put on, a high-tone signal of 1150 Hz is transmitted as the side tone signal to the user. Also, when the AMPS goes during its communication into the area making the communication difficult, or the communication of the AMPS is started, the above high-tone signal and the low-tone signal of 770 Hz are repeatedly generated and delivered to the user. This is accomplished by a circuit for generating the DTMF signal and the high/low-tone signal.
A prior-art circuit for generating the DTMF and high/low-tone signals generates the DTMF signal by using a semiconductor device designed for generating the DTMF tone which has been used in the ordinary telephone system, and uses as the low-tone signal of the high/low-tone signals the low-frequency of 770 Hz of the DTMF signals. Moreover, because the high-tone signal is not generated by the DTMF signal generator, it should be generated by a separate oscillator. However, since most of the commercial DTMF signal generators used in the ordinary telephone must solve the attenuation problem of the high-frequency signal resulting from the long-distance transmission of the DTMF signal through the communication line, the high-frequency group signals of the DTMF signals are emphasized by 2.7 dB(decibels) more than the low-frequency group signal. It is unsuitable to use such a conventional DTMF signal generator in the AMPS in view of the fact that the DTMF peak-frequency-deviation for each frequency component should be limited within 4.5.+-.10% in accordance with the provisions of the AMPS. Furthermore, a separate oscillator for generating the high-tone signal, as described above, is required in addition to the DTMF signal generator.
One of the conventional circuits for generating the DTMF signal to be easily fabricated in a semiconductor chip has used the frequency synthesizing method. Such a DTMF signal generator is disclosed in the U.S. Pat. No. 3,787,836 issued to Hagelbarger et al. on Jan. 12, 1974. According to the circuit dislcosed in the above patent, the frequency of master clock is divided into a fixed digital clock under the control of the keyboard. The train of the divided multi-frequency digital clock are used to make a phase shift into a fixed phase. This phase shifted component is properly synthesized to make a sine-wave signal to the multi-tone. However, in case of using the digital-to-analog synthesizing method has the digital square wave itself an odd higher-harmonic wave component. Consequently, the finally synthesized waveform has to have mentioned higher-harmonic wave component. If such a synthesized DTMF signal is used in the AMPS, transmission errors may occur.